This invention generally relates to reducing emission of nitrogen oxides from combustion systems. More particularly, the present invention relates to an apparatus for delivering an aqueous solution of a chemical agent to a flue gas for controlling emissions of nitrogen oxides from combustion systems.
Reduction of nitrogen emissions from exhaust of flue gases, before their release into the atmosphere, has been a prolific topic of discussion in the field of environmental aspects of energy production by combustion of fuel material. Because nitrogen oxide emissions are related to various environmental problems, the minimizing of nitrogen oxide release from combustion systems is an ongoing concern.
The process for controlling emissions of nitrogen oxides from combustion systems involves externally staged combustion and injection of a chemical agent. This process called advanced reburning can be improved or optimized if the chemical agent is introduced as an aqueous solution, and the sizes of the aqueous droplets are properly controlled. In its simplest form, returning is a process in which the fuel to air ratio in a combustion system is manipulated and controlled to reduce the amount of nitrogen oxides that reach the stack. The reburn process encompasses a combustion process in which the fuel and air are combined in a series of three major sub-processes to reduce the emissions of oxides of nitrogen from a combustion system.
Selective non-catalytic reduction (SNCR) is one approach used for controlling emissions of oxides of nitrogen. In this approach, a chemical agent is added to a combustion system where it reacts with oxides of nitrogen to reduce them to a molecular state. In its original form, SNCR involves the injection of ammonia into a flue gas duct of an air heater. As disclosed in U.S. Pat. No. 3,900,554 to Lyon, the contents of which are incorporated by reference herein. The SNCR technique described in the ""554 patent is effective only within a narrow temperature range, requiring that grids of tubing and injection nozzles be installed in the ductwork at different locations so that the location of ammonia injection could be moved in response to changes in combustor operation.
Alternate known approaches use chemical reagents such as, for example, urea and cyanuric acid to reduce emissions of nitrogen oxides from a combustion system. These compounds remained as solids at normal temperature, and thus were difficult to inject into combustion systems. While injection of aqueous solutions simplified the materials handling problems, new challenges were introduced. Key among these was the fact that the droplets require finite time to evaporate. Experimental evidence suggested that very little chemical agent became available for reaction until evaporation of water was completed.
In yet another approach, nozzles were specified and used based on the droplet sizes produced by each nozzle. This technique allowed the urea solution to be injected into the furnace in such a manner that the droplets would evaporate at the point of maximum effectiveness. The SNCR process, however, involves several drawbacks. In simple terms, the chemical agent, be it ammonia, urea, cyanuric acid, or other compounds, had to partially burn or react with oxygen in order to start the chain of events that leads to reduction of oxides of nitrogen. Therefore, a portion of the chemical agent is itself oxidized producing additional oxides of nitrogen.
In SNCR process, a number of intermediate species, known as free radicals, are also produced. These free radicals react with additional chemical agents to create the species that directly react with oxides of nitrogen leading to the production of molecular nitrogen. The SNCR process thus requires a delicate balance between the temperature and the chemical agents that are being introduced to reduce the oxides of nitrogen. If the temperature is just a little bit too high, the chemical burning is too fast, thus producing more oxides of nitrogen than it is able to reduce. If the temperature is just a little bit too low, the chemical agent does not burn fast enough, and not enough of the desired species are produced to permit reduction of oxides of nitrogen. In the latter situation, unreacted or partially reacted chemical agents can escape to the exhaust of the combustion system, and can become pollutants themselves.
The balancing process introduces practical difficulties in that, in some cases, the optimal location for injection of chemical agent is unavailable in a combustion system. In one approach, additional compounds were injected into the combustion system coincidentally with the chemical agent. Injection of these additional compounds produced additional free radicals at lower temperatures through a slow combustion process. These additional free radicals augmented free radicals produced through the burning of the chemical agent. At high temperatures, different compounds were added with the chemical agent. The action of these compounds was to either scavenge free radical species, or to promote the recombination of free radicals to form innocuous compounds such as water and carbon monoxide. In either case, the runaway reactions leading to oxidation of the chemical agents are suppressed and the desired reaction pathways leading to reduction of oxides of nitrogen are promoted.
FIG. 1 compares the effect of temperature on the SNCR process with and without the addition of additional compounds. Specifically, FIG. 1 indicates that by tailoring the overall chemistry of the combustion system 12 (FIG. 2), a certain amount of control may be afforded over the SNCR process, which may then become less sensitive to temperature.
Other approaches combined features of reburn technology and SNCR to produce a new process known as advanced reburning. In its simplest form, advanced reburning is a combination of reburn and SNCR, wherein the SNCR agent is added either with the reburn fuel or the overfire air. In advance reburning process, the chemical agent may be a well-known SNCR chemical agent, or may be another chemical compound, or mixture of compounds with similar chemical properties. Examples of chemical agents introduced into this advance reburning process include ammonium sulfate and ferrous ammonium sulfate.
In order to introduce sprays of aqueous chemical agent into a combustion system, conventional equipment used nozzle assemblies. In one approach, a pressure nozzle is used wherein the liquid behind the nozzle exit is under significant pressure. The pressurized liquid leaves the nozzle in a thin sheet that subsequently breaks down into filaments and ligaments eventually forming droplets. Another approach adopted a twin fluid nozzle wherein a supply of a second fluid, usually a gas such as, for example, air, is pressurized behind the nozzle. As the gas expands and accelerates across the nozzle it forms a momentum boundary layer across the surface of the liquid. Shear forces in this boundary layer transfer momentum from the gas to the liquid, resulting in surface instabilities that create droplets. In both these approaches, the geometry of the nozzle dictates the size distribution of the droplets produced. In a twin fluid nozzle some control over the droplet size distribution can be achieved by varying the pressure of the second fluid. In general, the average drop size is larger than the design drop size for fluid pressures below the design pressure. Also, increasing the average drop size tends to result in broadening the overall drop size distribution, and a significant fraction of the fine droplets continue to be produced.
Thus, with conventional equipment for introducing sprays of aqueous solutions of chemical agents into a combustion system, there will be a broad range of drop sizes, with a significant fraction of the droplet population being quite small and susceptible to evaporation in a very short time frame. This makes control over the droplet evaporation time difficult, and reduces the effectiveness of the droplet injection process.
Accordingly, an apparatus and method for delivering an aqueous solution of a chemical agent to a flue gas within a combustion system, while controlling the size and distribution of aqueous droplets. The method and apparatus reduces the emissions of nitrogen oxides from combustion system by injecting a fluid chemical agent to a flue gas within the combustion system. The apparatus includes at least one nozzle assembly having an array of openings. The nozzle assembly design is based on the Raleigh-Weber phenomenon, and produces fluid droplets with an average droplet size and narrow size distribution that is advantageous for the advanced reburning process by injecting the chemical agent(s) into overfire air in the combustion system. A fluid flow controller is used to control fluid parameters such as, for example, flow rate, pressure, thereby controlling the diameter of fluid droplets issuing from the nozzle.
An excitation mechanism is also provided to vibrate the nozzle assembly or acoustically excite the fluid being delivered to the nozzle assembly. The excitation mechanism provides additional control over the average droplet size and distribution. The openings of the nozzle may be arranged in linear, circular or elliptical fashion to eliminate the possibility of droplet collisions which may lead to agglomeration of fluid droplets.
The formation of fluid droplets is represented by the following equation (referred to as Raleigh-Weber equation):   D  =      1.89    ⁢                  D        j            ⁡              [                  1          +                                    3              ⁢                              μ                l                                                                    (                                                      σρ                    l                                    ⁢                                      D                    j                                                  )                                            1                /                2                                                    ]            
where,
D=droplet diameter
Dj=jet diameter
xcexcl=liquid viscosity
xcfx81l=liquid density
"sgr"=liquid surface tension
In one aspect, the apparatus for controlling the emissions of nitrogen oxides from a combustion system includes at least one nozzle assembly having an array of openings for delivering droplets of a fluid chemical agent to a flue gas within the combustion system; a first controller operatively coupled to the at least one nozzle assembly for controlling the size and distribution of each droplet; a second controller operatively coupled to the at least nozzle assembly for providing additional control over the size and distribution of each droplet; and the array of openings arranged for deflecting the droplets to prevent droplet collisions. The array openings are preferably disposed in a circular arrangement. The array openings may also be disposed in a linear or an elliptical arrangement. Preferably, the longitudinal axes of the openings are aligned to prevent the velocity vectors of the fluid issuing from the nozzle from intersecting. Droplet from the array of openings preferably exit in a divergen pattern to prevent droplet collisions. The apparatus further includes a delivery system for supplying fluid to the array of openings, the delivery system designed to generate a velocity vector of the fluid approaching each opening to be normal to the surface of a respective opening. The amount of fluid delivered through each opening of the array is approximately equal for openings of equal diameter. Preferably, the diameter of each droplet is in the range of 100-1200 microns. The diameter of the droplets is controlled by the Raleigh-Weber equation as noted above. The second controller may be a frequency controller.
In another aspect, apparatus for controlling the emissions of nitrogen oxides from combustion systems comprising at least one nozzle assembly having an array of openings for delivering droplets of a fluid chemical agent to the combustion system, the openings being arranged to prevent droplet collisions, and wherein the diameter of each droplet is controlled by the Raleigh-Weber equation. A controller is operatively coupled to the at least one nozzle assembly for exciting the fluid delivered to the nozzle and providing additional control over the droplet size and distribution.
In another aspect, in a gas turbine of the type having an combustor, at least one nozzle assembly having an array of openings, a method for controlling the emissions of nitrogen oxides from the combustor, comprises arranging the openings of the array to prevent droplet collisions; delivering droplets of fluid chemical agent to the combustor; controlling the fluid delivered to each nozzle; and exciting the fluid delivered to the at least one nozzle for controlling the droplet size and distribution. The method further includes aligning longitudinal axes of the openings to prevent the velocity vectors of the liquid issuing from each opening from intersecting. Also, fluid is delivered to each opening so as to make the velocity vector of the fluid approaching each opening normal to the surface of a respective opening. The volume of fluid delivered through each opening of the array is approximately equal for openings of equal diameters.
In a further aspect, a method for controlling the emissions of nitrogen oxides from a combustion system including arranging the openings of a nozzle to prevent droplet collisions; controlling the fluid delivered to the nozzle; exciting the fluid delivered to the nozzle for controlling the droplet size and distribution; and delivering droplets of fluid chemical agent to the combustion system.